PEGylated Mutated Clostridium botulinum Toxin

ABSTRACT

The invention relates to a modified botulinum toxin comprising a natural heavy chain and a modified light chain, characterized in that the modification of the light chain resides in that it comprises (i) an extension of the chain on its N-terminus which has the structure -(C) n -(tag) m -(X) l - in the direction from the N- to the C-terminal end, wherein
         C represents a cysteine residue,   tag represents any tag and   X represents the residue of any naturally occurring amino acid,   n represents an integer from 1 to 50,   m represents 0 or 1, and   l represents 0 or an integer from 1 to 50,
 
and in that (ii) at least one of the cysteine residues in the extension of the chain is coupled to at least one chain of PEG.

This application is a continuation of U.S. application Ser. No. 13/624,493 filed Sep. 21, 2012, which is a divisional of U.S. application Ser. No. 13/185,925 filed Jul. 19, 2011, now U.S. Pat. No. 8,298,550, which is a divisional of U.S. application Ser. No. 12/282,601, now U.S. Pat. No. 8,003,601, filed as a national phase application under 35 U.S.C. §371 of International Application No. PCT/EP2007/002296 filed Mar. 15, 2007, which claims priority to European Patent Application No. EP 06 005 300.6 filed Mar. 15, 2006. The entire text of each of the above-referenced disclosures is specifically incorporated herein by reference without disclaimer.

The present invention relates to modified botulinum toxins (BoNT), which have enhanced stability and thus a prolonged therapeutic duration of action as compared to the corresponding native botulinum toxins. Furthermore, the present invention relates to pharmaceutical compositions comprising these modified botulinum toxins. Finally the present invention relates to nucleic acids, which encode these modified botulinum toxins.

BACKGROUND OF THE INVENTION

Clostridium botulinum is an anaerobically growing, sporulating bacterium, which produces a highly toxic protein. This so-called botulinum toxin is the cause of botulism, a food poisoning, which without the use of intensive care measures can lead to the death of botulism patients. Seven serotypes are distinguished (type A-G, shortly termed BoNT/A, BoNT/B, etc.) that have a similar amino acid sequence, but induce a different antibody response. The toxins (hereinafter also referred to as neurotoxins and botulinum toxins) consist of two functional chains, the light (˜50 kDa) and the heavy chain (˜100 kDa), which are generated by proteolytic cleavage of the single-chain precursor protein. Other strains do not possess the corresponding protease, therefore the cleavage into the chains takes place in the gastrointestinal tract of the patients (e.g. by trypsin). In the double-chain form the subunits (i.e., the heavy and the light chain) are interconnected via disulfide bridges (for example, in addition there exists an intramolecular disulfide bridge in BoNT/A, i.e., between two cysteine residues of the heavy chain).

Under acidic conditions in vivo the pure neurotoxins do not exist in free form, but form complexes with other clostridial proteins, the so-called (Clostridium botulinum) toxin complexes. Different proteins, inter alia with hemagglutinating properties, are involved in these complexes. The composition of the complexes is different between serotypes. The integration into the complex protects the neurotoxin during gastrointestinal passage. These other clostridial proteins (the complexing and complex proteins, respectively) possibly play also a role in the resorption of the neurotoxin. Thus, the incorporation in the complex causes the neurotoxin to be orally bioavailable and to thus constitute a food poison. The target location of the neurotoxins is at the motor endplate, where the muscle is activated by the nerve. The motoneuron releases acetylcholine for activation of the muscle. This release is inhibited by botulinum toxin. The inhibitory effect takes place in 3 sequential steps: binding, translocation, proteolysis. The heavy chain of botulinum toxin binds highly specific to the motoneuron and is subsequently taken up into the nerve cell by endocytosis. Upstream of the binding domain, which is located at the C-terminal end of the heavy chain, there is the translocation domain in the N-terminal portion of the heavy chain, which transports or, rather, facilitates the translocation of the light chain into the cytosol by an as yet unknown mechanism. In the cytosol the light chain becomes active as a protease cleaving highly specific so-called SNARE proteins. The proteolytic specificity of the individual botulinum toxin types is summarized in Table 1. These SNARE proteins are responsible for the fusion of the acetylcholine-loaded secretory vesicles with the cell membrane of the motoneuron. The proteolytic cleavage of one of these SNARE proteins inhibits the formation of a fusion complex and thus further release of acetylcholine. The affected muscle is no longer activated. Previously hyperactive muscles become paralyzed.

TABLE 1 SNARE protein Botulinum Toxin Substrate of protease Cleavage site in SNARE Type activity sequence from rat Type A SNAP 25 EANQ¹⁹⁷ RATK Type B VAMP 2 GASQ⁷⁶ FETS Type C Syntaxin DTKK²⁵⁴ AVKY SNAP 25 ANQR¹⁹⁸ ATK Type D VAMP 2 RDQK⁶¹ LSED Type E SNAP 25 QIDR¹⁸⁰ IMEK Type F VAMP 2 ERDQ⁶⁰ KLSE Type G VAMP 2 ETSA⁸³ AKLK

This mechanism of action is taken advantage of in the therapy of a multitude of muscle disorders and spasms, respectively, characterized by an uncontrolled release of acetylcholine (e.g., blepharospasm, torticollis, spasticity) Extremely low amounts of the neurotoxin (in the pg to ng range) are injected in the hyperactive muscle for therapy of dystonia. The neurotoxin diffuses to the motor endplate and reaches the cytosol of the neuron to inhibit the acetylcholine release there. The muscle is paralyzed after 1-2 days.

Various facial wrinkles are formed by cramping of muscles lying beneath the skin, thus also through uncontrolled release of acetylcholine. Botulinum toxins find cosmetic utilization in this context: wrinkles will be removed for about 3 months through injection of extremely low amounts of botulinum toxin.

At present four preparations containing botulinum toxins have received drug-regulatory approval: Botox® (Allergan), Xeomin® (Merz), Dysport® (Ipsen), and NeuroBloc® (Solstice Neurosciences). Botox®, Xeomin®, and Dysport® are lyophilisates of botulinum toxin type A (as complex, neurotoxin and complex, respectively), Botox® and Xeomin® with 100 units per injection vial each, Dysport® with 500 units. NeuroBloc® contains botulinum toxin type B (as complex) with 5,000 and 10,000 units, respectively, in liquid formulation.

Except for NeuroBloc the preparations are available as lyophylisates, which are reconstituted with physiological saline and are injected in the respective muscles in matched doses depending on preparation and indication. The treated muscle will be paralyzed within 48 h. The effect lasts about 3 month, thereafter a further injection must be carried out, if the muscle should remain paralyzed further, i.e. the dystonia is to be treated. Up to now it has not unambiguously elucidated, which processes control the decrease of the effect. As long as the light chain is active as protease, the appropriate SNARE protein is cleaved (e.g., SNAP 25 through the light chain of neurotoxin type A). Accordingly, the fusion of the secretory vesicles with the plasma membrane and thereby the release of acetylcholine will be inhibited under these conditions, the muscle remains paralyzed. If it were possible to maintain the protease activity of the light chain for an extended time period in the cell, then the duration of action of an appropriate drug would be extended also

In contrast to many low molecular active substances active protein substances are characterized by a significantly lower stability. The half life (HL) of some active protein substances in the circulating blood amounts to only a few minutes, so that the (therapeutic) duration of action is strongly restricted and injection must be repeated in short intervals. The HL can be extended, if one is successful in protecting the protein against degradation and elimination processes. One theoretically possible way exists especially for eukaryotic proteins in a higher glycosylation (more carbohydrate moieties) and in adapting the carbohydrate structures to the structures of human glycoproteins, respectively. Another path that has been taken in a series of approved active substances is the coupling of the protein with polyethylene glycol (PEG). PEG can be covalently bonded to the residues of various amino acids, e.g., to lysine (amino function) or cysteine residues (SH function). PEG enhances the molecular weight of the protein without creating immunogenic structures that induce the generation of antibodies to the active substance. To the contrary: the PEGylation reduces the immunogenicity of the active substance. The protein is eliminated more slowly by the increase of the molecular weight and a significant increase in HL is achieved. For maintaining a certain required serum level, the drug has to be injected less often.

PEGylated active protein substances are already processed in some approved drugs (see Table 2). The employment of the partly small proteins (e.g., interferon α 2a: Mr=19.3 kDa) in the original form, i.e. not modified, has shown that the proteins are very rapidly eliminated from the serum. The PEGylation gave rise to a markedly increased molecular weight and thus to a substantially longer half life in the serum. Thus, for example, the serum half life for interferon α 2a is 9 h; PEGylation with a 40 kDa PEG chain drastically increases the molecular weight and extends the half life from 9 to 72 h.

TABLE 2 Trade name Starting compound Coupling of PEG Pegasys interferon α 2a branched PEG-N-hydroxysuccinimide; Coupling to 4 lysine residues Neulasta G-CSF PEG-aldehyde; Coupling to N-terminal methionine Peglutron interferon α 2b Succinimidyl carbonate-PEG; Coupling to histidine and lysine residues Somavest growth hormone 4-6 PEG; antagonist Coupling to lysine residues and N- terminus Oncaspar Asparaginase N-hydroxysuccinimide activated PEG

However the linkage with one or more PEG chains is subject to restrictions:

-   1. Preferably the PEG chain diminishes the biological activity of     the modified protein (in comparison with the unmodified native     protein) not at all or only slightly (in accordance with the     invention it is understood that slightly diminished biological     activity of the modified protein corresponds to at least 20%,     preferably to 30-40% or 50-70% or even to 75-95% of the biological     activity of the unmodified native protein). A diminished activity is     tolerable in many cases: e.g. the antiviral activity of PEGylated     interferon is 25-35% of the non-PEGylated interferon α 2b. PEGylated     interferon α 2a even possesses only 1-7% of the activity of the     non-PEGylated form. -   2. As a multitude of therapeutically employed proteins deploy their     activity through the binding to a specific receptor, preferably the     PEGylation does not affect, or only slightly affects, the     interaction with the receptor (e.g., the interaction can be affected     directly by steric hindrance at the binding domain or by alterations     of the spatial structure of the protein that have an effect on the     binding domain and hence on binding). -   3. When the pharmacological effect of the therapeutic protein is     (also) mediated through an enzymatic activity (as for instance with     asparaginase), preferably the enzymatic activity is not, or only     slightly, reduced through the PEGylation.

Preferably the PEGylation of botulinum toxin accomplishes these three criteria. At the same time the modification of the botulinum toxin with PEG preferably influences neither (a) the binding domain of the heavy chain nor (b) the enzymatic activity of the light chain, i.e., the PEG chain preferably does not inhibit the interaction of the catalytic domain from the light chain with the substrate (SNARE Protein). In contrast to other proteases, that cleave short peptides, botulinum toxins require longer peptides as substrates. For instance, a peptide which serves as a substrate for botulinum toxin type B preferably has a sequence of about 40 amino acid residues of the SNARE protein VAMP 2. Peptides with shorter SNARE sequences will also be cleaved, but with substantially lower efficiency. The cleavage domain of the light chain of the botulinum toxin, which has a length comparable to the recognition sequence of about 40 amino acid residues, is preferably not affected by the PEG chain. Moreover, it has to be considered, that besides the cleavage domain responsible for the direct contact of the substrate (SNARE protein and peptide with the SNARE sequence of about 40 amino acid residues, respectively) with the light chain, additional contact sites with sequences on the light chain located distantly to the catalytic domain are needed for optimal activity of botulinum toxin. It has been demonstrated that five additional contact sites for its substrate SNAP 25 are localized on the light chain of botulinum toxin type A: 4 α exosites (AS 102-113, 310-321, 335-348, 351-358) and one β exosite (AS 242-259). Preferably, the contact is not or only marginally impeded through a conjugation of the light chain with PEG. Moreover, the C-terminal part of the heavy chain, the translocation domain, must be operable, i.e., it must ensure that the light chain is transported from the endosomes into the cytosol. This transport process that is absolutely necessary for the action can also be inhibited through the steric hindrance of a PEGylated light chain especially as the translocation domain possibly forms a pore in the endosomal membrane through which a “bulky” PEGylated light chain might not be channeled through.

Coupling of PEG to botulinum toxin is reported in a U.S. patent application (2002/0197278). The coupling serves to diminish the antigenicity and immunogenicity, respectively as well as to enhance the molecular weight for reducing the diffusion. For the selection of the appropriate sites (antigenic determinants) and amino acid residues, respectively, for the PEGylation, reference is being had to the paper of Bavari et al. (Vaccine 16: 1850-1856, 1998). In this paper sequences of the botulinum toxin heavy chain that induce neutralizing antibodies are presented. In the aforementioned patent application it is only stated that (1) the PEGylation should be carried out at, respectively close to one site or at, respectively, close to the sites, which act(s) as an important epitope(s), but which are remote from the catalytic domain (i.e. remote from the light chain) and that (2) PEG may be conjugated to the free terminal carboxy or amino groups or at the amino groups of lysine side chains. (3) As additional alternative for the insertion of PEG into the toxin it is suggested to use the SH groups of naturally occurring or specially inserted cysteine residues; however, the paper advises against this alternative (3), as disulfide bridges between the heavy and the light chain of the botulinum toxin play a role in the spatial configuration of the molecule. There is no example given that discloses the structure of the PEGylated neurotoxin or that discloses on which amino acid residue(s) a PEG molecule of a certain length was attached.

In a further patent application (WO 02/40506) relating to the change in stability, the insertion, the modification or the removal of sites for the in vivo glycosylation, in vivo phosphorylation and primarily the in vivo myristoylation in the botulinum toxin are suggested in order to optionally either enhance or decrease the stability of the botulinum toxin. A whole series of potential modification sites are specified which are located at a significant distance to the N- and C-terminal ends of the neurotoxin light chain. Additional sequences are to be inserted into the polypeptide chain, where carbohydrate chains or phosphate and myristoyl moieties, respectively, are coupled at the light chain by cellular enzymes. Information regarding an accordingly modified neurotoxin or its preparation is however missing.

In a further U.S. patent application (2003/0027752) a peptide residue with a so-called leucine motif (e.g., XEXXXLL) is inserted into the neurotoxin or into the light chain in order to increase the stability of the light chain within the nerve cell. The configuration of the light chain with this motif ensures that it is localized in the vicinity of its substrate at the membrane. Moreover, a so-called “tyrosine based motif” (YXXHy, Y=tyrosine, Hy=hydrophobic amino acid) is set forth that, after insertion in the light chain, is to enhance its persistency. Finally this patent application suggests a modified botulinum toxin type A, in which the light chain is mutated (alanine to leucine at the positions 427 and 428).

In view of the above described prior art it was an object of the inventors to provide an additional or precisely described form of stabilization for any type of botulinum toxin preferably, however, for type A, B, and C1. Along with this the object of the inventors was to provide stable variants/analogs of the natural botulinum toxins which in comparison to the respective unmodified botulinum toxins have an increased in vivo stability. This means, firstly, that the biological activity (according to the invention biological activity is defined as total activity comprising the enzymatic/catalytic activity of the light chain as well as the required neurotoxin binding to the target cell and the translocation of the light chain into the target cell) of the botulinum toxin variant/analog shall be at most marginally (according to the above definition), and preferably not at all, decreased and, secondly, that, in spite of its modification, the light chain is translocated to its site of action, the cytosol of the motoneuron.

In contrast to the already aforementioned US 20020197278 the objective forming the basis of the present application does not aim to block antigenic determinants, to decrease the antigenicity of the toxins or to restrict their diffusion away from the injection site.

The inventors of the present application surprisingly found, that the light chain of botulinum toxins can be specifically PEGylated at its N-terminus via insertion of at least one cysteine residue without simultaneously impairing or even inhibiting the biological activity (according to the definition given above) of the botulinum toxins. Such a PEGylated botulinum toxin is characterized by a surprisingly higher in vivo stability (significant increase of HL and therewith an extended (pharmacological) duration of action).

The (therapeutic) duration of action of the natural botulinum toxins in the patient depends on the serotype. Botulinum toxin type A is characterized by the longest duration of action of about 3 month. The duration of action of botulinum toxin type C is of similar length as that of type A, whereas botulinum toxin type B has a shorter duration of action. The effect of botulinum toxins type E and F lasts only about 2 weeks in each case. The short duration of action of these two types does not allow their clinical application for the treatment of dystonia. The present invention allows (1) the clinical application of all botulinum toxins, even those having so far short-term activity, and (2) a more advantageous therapy with the already therapeutically utilized type A and B toxins, as these need not be administered every three month, but, e.g., only every six month.

DESCRIPTION OF THE FIGURES

FIG. 1: Summary of the oligonucleotides (SEQ ID NO:1 to 14) which have been employed in the cloning of the recombinant toxins and toxin fragments. Recognition sequences for restriction endonucleases are underlined. The long sequences with the SEQ ID NOs: 16 and 15 show examples for a recombinant (mutated) botulinum neurotoxin type A with attached cysteine residue N-terminal to a histidine tag (consisting of 10 histidine residues) or a DNA that encodes it. The proline residue at the N-terminus of the native toxins (position 1) that is monocystronically expressed and translated was replaced by an alanine residue to create a cleavage site in the multi-cloning site (MCS) of the vector. The vector comprises the coding sequence for the His-tag precisely in the 5′-vicinity of this MCS. In addition a sequence which is recognized by E. coli cells has already been inserted in place of the native loop between the light and the heavy chain whereupon the native pre-peptide (N-light chain-loop-heavy chain-C) is already cleaved into the active two-chain neurotoxin without the addition of exogenous proteases and obtained as such during the recombinant production of the neurotoxin.

FIG. 2: Analysis of the PEGylation and control batches of C-H₁₀-BoNT/A (Example 5) in SDS polyacrylamide gels under non-reducing conditions. Lane 1: molecular weight marker; Lane 2: control batch; Lane 3: PEGylation batch.

DESCRIPTION OF THE INVENTION

In order to solve the stated object (see above) the inventors have developed modified botulinum toxins. One aspect of the invention therefore relates to a modified botulinum toxin comprising a natural heavy chain and a modified light chain, wherein the modification of the light chain is that it comprises (1) an extension of the chain on its N-terminus which has the following structure in the direction from the N- to the C-terminal end: -(C)_(n)-(tag)_(m)-(X)_(l)-, wherein

-   -   C represents a cysteine residue,     -   tag represents any tag, e.g. a Strep-tag or a His-tag, and     -   X represents the residue of any naturally occurring amino acid,     -   n represents an integer from 1 to 50,     -   m represents 0 or 1, and     -   l represents 0 or an integer from 1 to 50,

and (2) that at least one of the cysteine residues in the extension of the chain is coupled to at least one chain of PEG. Such a modified botulinum toxin is hereinafter also referred to as PEGylated mutated botulinum or neurotoxin.

According to a preferred embodiment, the following conditions apply:

-   -   n=1, 2 or 3, m=0 or 1, l=0 or l≠0,     -   n=1, m=1, l=0; n=2, m=1, l=0; n=3, m=1, l=0;     -   n=1, m=0, l=0; n=2, m=0, l=0; n=3, m=0, l=0;     -   n=1, m=1, l≠0; n=2, m=1, l≠0; n=3, m=1, l≠0;     -   n=1, m=0, l≠0; n=2, m=0, l≠0; n=3, m=0, l≠0;

wherein the toxins per molecule are coupled to one, to two or to three PEG molecules, depending on whether n=1, 2 or 3.

Thus preferably those modified botulinum toxins (especially of the types A, B, and C1) fall within the aforementioned modified botulinum toxins of the present invention whose light chain is modified in such a way that it comprises an extension of the chain, wherein the extended chain has one of the following sequences:

-(C)₁-(tag)₁-(X)₀-, -(C)₂-(tag)₁-(X)₀-, -(C)₃-(tag)₁-(X)₀-, -(C)₄-(tag)₁-(X)₀-, -(C)₅-(tag)₁-(X)₀-,

-(C)₁-(tag)₁-(X)₁-, -(C)₂-(tag)₁-(X)₁-, -(C)₃-(tag)₁-(X)₁-, -(C)₄-(tag)₁-(X)₁-, -(C)₅-(tag)₁-(X)₁-,

-(C)₁-(tag)₁-(X)₂-, -(C)₂-(tag)₁-(X)₂-, -(C)₃-(tag)₁-(X)₂-, -(C)₄-(tag)₁-(X)₂-, -(C)₅-(tag)₁-(X)₂-,

-(C)₁-(tag)₁-(X)₃-, -(C)₂-(tag)₁-(X)₃-, -(C)₃-(tag)₁-(X)₃-, -(C)₄-(tag)₁-(X)₃-, -(C)₅-(tag)₁-(X)₃-,

-(C)₁-(tag)₁-(X)₄-, -(C)₂-(tag)₁-(X)₄-, -(C)₃-(tag)₁-(X)₄-, -(C)₄-(tag)₁-(X)₄-, -(C)₅-(tag)₁-(X)₄-, etc.

wherein in any of the above listed 25 preferred embodiments m can also be 0 instead of 1, and/or in each case all cysteine residues occurring in the extension of the light chain are also PEGylated.

Of course l can also be any integer from 11 to 50 or above 50, most preferred above 100 or above 250. However, the greater l is, the longer the light chain becomes without the enzymatic/catalytic activity or the (total) biological activity (according to the above definition) being compromised by a specific upper limit for the length of the light chain. For reasons of practicability, however, an upper limit to l of 10-20 is preferred so that preferred values for l are in the range of 1-10, unless l is 0, which is especially preferred.

Appropriate considerations also apply to n, wherein according to the invention its upper limit has been set to 50 for practical and economical reasons. Preferably n is in the range of 1-10, more preferred 1-5, so as not to insert, or not to have to insert, too many PEG molecules (as all inserted cysteine residues are preferably PEGylated), and not to deprive the resulting PEGylated mutated botulinum/neurotoxin of its biological activity according to the aforementioned definition. This can easily occur, if too many cysteine and PEG residues are inserted or inserted at incorrect positions, as the light chain, perhaps despite the binding of the toxin to the target cell, is not translocated into the target cell.

The structure of botulinum toxin type A was published by Lacy & Stevens 1998 (Nat. Struct. Biol. 5, 898-902), the structure of botulinum toxin type B by Swaninathan & Eswaramoorthy (Nat. Struct. Biol 7, 693-699 (2000)). Therefore the structure of the light chains is also known and one can determine which region of the heavy and the light chain, respectively are at the protein surface and thus may be suitable for a coupling with PEG. The frequently chosen procedure of binding an appropriately activated PEG (e.g., PEG-succinimidylpropionate) to the ε-amino group of lysine residues did not seem promising to the inventors. An activated PEG can react with numerous lysine residues—even in the binding region of the heavy chain and this leads experimentally to a drastic inactivation.

Instead, the inventors identified amino acid residues of the light chain which are suited initially to be replaced by at least one cysteine residue and subsequently to be PEGylated on the at least one inserted cysteine residue. These modified botulinum toxins, hereinafter to be characterized in more detail, which are also a preferred embodiment of the present invention and have as conjugates with PEG a sufficient biological (including enzymatic/catalytic) activity (which by definition corresponds at least to 20%, preferably to 30-40%, to 50-70% or even to 75-95% of the biological activity of the unmodified protein) with simultaneously increased stability (in comparison to the corresponding native neurotoxins) and will hereinafter also be referred to as PEGylated mutated botulinum or neurotoxins of the present invention.

These latter modified botulinum toxins of the present invention are also conjugates of mutated botulinum toxins with PEG. These modified botulinum toxins are also coupled to PEG via separately inserted cysteine residues. For this purpose at least one, but optionally also 2, 3, 4, 5, 10 or even all 20 of the first 20 amino acid residues of the N terminal end of the light chain of the respective botulinum toxin is in each case replaced by a cysteine residue. These modified botulinum toxins also comprise a natural heavy chain and a modified light chain, wherein the modification of the light chain is such that at least one up to maximally 20 of the amino acid residues occurring naturally at the N terminus are mutated to a cysteine residue. If applicable they comprise an additional N terminal extension, such that the sequence of the light chain has the following structure in the direction from the N-terminus to the C-terminus: -(tag)_(m)-(X)_(l)-BoNT(X1-20C), wherein

-   -   C represents a cysteine residue,     -   tag represents any tag, e.g. a Strep-tag or a His-tag, and     -   X represents the residue of any naturally occurring amino acid,     -   m represents 0 or 1, and     -   l represents 0 or an integer from 1 to 50.

At least one of the maximally 20 cysteine residues at the N-terminus is coupled to at least one chain of PEG.

Hence there result mutants for BoNT/A, which are characterized by at least one—and not more than twenty—of the following replacements of amino acid residues, such that a PEGylation can occur at the inserted cysteine residues. P1C, F2C, V3C, N4C, K5C, Q6C, F7C, N8C, Y9C, K10C, D11C, P12C, V13C, N14C, G15C, V16C, D17C, I18C, A19C, Y20C

According to a preferred embodiment, the following conditions apply:

-   -   m=1, l=0, only one out of the 20 N-terminal amino acid residues         is replaced by a cysteine residue, more preferable only the         residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10;     -   m=0, l=0, only one out of the 20 N-terminal amino acid residues         is replaced by a cysteine residue, more preferable only the         residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10;     -   m=1, l≠0, only one out of the 20 N-terminal amino acid residues         is replaced by a cysteine residue, more preferable only the         residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10;     -   m=0, l≠0, only one out of the 20 N-terminal amino acid residues         is replaced by a cysteine residue, more preferable only the         residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10;     -   m=1, l=0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2         and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6;     -   m=0, l=0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2         and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6;     -   m=1, l≠0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2         and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6;     -   m=0, l≠0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2         and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6;     -   m=1, l=0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or         5 and 6;     -   m=0, l=0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or         5 and 6;     -   m=1, l≠0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or         5 and 6;     -   m=0, l≠0, only two out of the 20 N-terminal amino acid residues         are replaced by a cysteine residue, more preferable only the         residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or         5 and 6,

wherein the toxins per molecule are coupled to one or two PEG molecules, depending on whether only one or two amino acid residues have been replaced by (one) cysteine residue(s) at the N-terminus.

Therefore preferably those botulinum toxins (especially of the types A, B, and C1) fall within the aforementioned modified botulinum toxins whose light chain is modified in such a way that it comprises an extension of the chain at its N-terminus, wherein the N-terminus of the extended chain has one of the following sequences:

-(tag)₁-(X)₀-BoNT(P1C), -(tag)₁-(X)₀-BoNT(F2C), -(tag)₁-(X)₀-BoNT(V3C), -(tag)₁-(X)₀-BoNT(N4C), (tag)₁-(X)₀-BoNT(K5C),

-(tag)₁-(X)₁-BoNT(P1C), -(tag)₁-(X)₁-BoNT(F2C), -(tag)₁-(X)₁-BoNT(V3C), -(tag)₁-(X)₁-BoNT(N4C), -(tag)₁-(X)₁-BoNT(K5C),

-(tag)₁-(X)₂-BoNT(P1C), -(tag)₁-(X)₂-BoNT(F2C), -(tag)₁-(X)₂-BoNT(V3C), -(tag)₁-(X)₂-BoNT(N4C), -(tag)₁-(X)₂-BoNT(K5C),

-(tag)₁-(X)₃-BoNT(P1C), -(tag)₁-(X)₃-BoNT(F2C), -(tag)₁-(X)₃-BoNT(V3C), -(tag)₁-(X)₃-BoNT(N4C), -(tag)₁-(X)₃-BoNT(K5C),

-(tag)₁-(X)₄-BoNT(P1C), -(tag)₁-(X)₄-BoNT(F2C), -(tag)₁-(X)₄-BoNT(V3C), -(tag)₁-(X)₄-BoNT(N4C), -(tag)₁-(X)₄-BoNT(K5C), etc.

wherein in any of the above listed 25 preferred embodiments m can also be 0 instead of 1, and/or in each case all N-terminally inserted cysteine residues are also PEGylated.

Of course, l can also be any integer from 11 to 50 or above 50, most preferred above 100 or above 250. However, the greater I, the longer the light chain without the enzymatic/catalytic activity or the (total) biological activity of the neurotoxin becoming compromised with regard to the above mentioned definition by a specific upper limit for the length of the light chain. For reasons of practicability, however, an upper limit of l of 10-20 is preferred, so that preferred values for l are in the range of 1-10, unless l is 0, which is especially preferred.

Unless explicitly specified otherwise, the following explanations apply to the modified botulinum toxins of the present invention, regardless whether it is the variant with inserted cysteine residue(s) in the N-terminal extension of the light chain or the variant with inserted cysteine residue(s) at the N-terminal end of the light chain.

Preferably, the modified botulinum toxin is a modified botulinum toxin derived from BoNT/A, BoNT/B, or BoNT/C1, but it also can be a botulinum toxin of the types D, E, F, or G. It is also preferred that on the one hand all artificially inserted cysteine residues (preferably 1-10 cysteine residues are inserted) comprise at least one PEG chain, but on the other hand none of the naturally occurring cysteine residues of the heavy and light chains of the botulinum toxin is PEGylated.

It is self-explanatory to a person skilled in the art, that the relevance of the tag (m=1) resides in the easier purification of the recombinantly (e.g., in E. coli) produced modified botulinum toxin of the present invention. Thus, the tag is not used to increase the stability of the neurotoxin or its biological activity, but allows the simplified and almost quantitative isolation of the modified botulinum toxin from the bacterial culture.

Regarding the question of an appropriate choice of n (in case of the variant with inserted cysteine residue(s) in the N-terminal extension of the light chain) or the number and position of the amino acid residues to be replaced by cysteine residues (in case of the variant with inserted cysteine residue(s) at the N-terminal end of the light chain) one has to consider that preferably only as many cysteine residues are inserted as PEGylations are to be carried out (the same applies to the case, where l≠0.0 and at least one amino acid residue X is a cysteine residue). All of these inserted cysteine residues can preferably be PEGylated, and completely at that, without, and that is the surprising finding of the inventors, even one of the cysteine residues occurring naturally in botulinum toxin being PEGylated on the heavy or on the light chain (besides one intra- and intermolecular disulfide bridge each, botulinum toxin type A e.g. exhibits three additional cysteine residues in the heavy chain (C₇₉₁, C₉₆₇, C₁₀₆₀) as well as two additional cysteine residues in the light chain (C₁₃₄ and C₁₆₅)). In this manner, a consistent (homogeneous) product in the form of a PEGylated mutated botulinum toxin can be obtained. Furthermore, it is reasonable to avoid too many cysteine or PEG residues being inserted or inserted at incorrect positions for an additional reason. This is so because (i) the toxin possibly may then become too bulky to bind to the target cell and/or (ii) the light chain is, perhaps despite binding of the toxin to the target cell, not sufficiently translocated into the target cell. In other words, the insertion of only one cysteine residue in this or that variant and its PEGylation routinely accomplishes the purpose of the invention and is therefore especially preferred.

Accordingly, the PEGylated mutated botulinum/neurotoxins in accordance with the present invention are, like the corresponding natural (native) botulinum/neurotoxins, biologically and enzymatically (i.e., catalytically) active or exhibit in the sense of the definition given above not more than a marginally decreased biological and enzymatic/catalytic activity, but are more stable, partly even substantially more stable, than their natural precursor toxins from which they were derived. Furthermore, a PEGylated mutated botulinum toxin is preferred, whose PEGylated mutated (modified) light chain exhibits a higher stability in the cytosol of the motoneuron than the unmodified light chain of the corresponding native botulinum toxin.

As described above, the PEGylation of the light chain results in an increased stability compared to the unmodified light chain. In accordance with the invention the PEG chain is attached to the light chain thus, that, firstly, its enzymatic activity is unchanged or at most marginally decreased (according to the definition given above), and, secondly, the modified light chain like the unmodified chain is translocated into the cytosol of the motoneuron.

The His-tag as well as other tags, e.g., the Strep-tag, allows the straightforward isolation of the mutated neurotoxin from the lysate of transformed bacteria (e.g., E. coli). At the DNA level it is most simply attached in 5′-direction to the coding region of the neurotoxin gene. In the case of the His-tag the isolation occurs by use of affinity chromatography on Ni-NTA-sepharose. In the next step, the mutated neurotoxin that is isolated in this way is coupled to activated PEG. A series of activated PEG derivatives, e.g., PEG-maleimide, PEG-vinylsulfone, PEG-iodoacetamide, and PEG-orthopyridyl-disulfide are provided by Nektar Therapeutics, and instructions for the PEGylation are supplied. In accordance with the invention these PEG derivatives may comprise different chain lengths: e.g., PEG derivatives with molecular weights of 5,000 Dalton, 10,000 Dalton, 20,000 Dalton, and 30,000 Dalton are commercially available and to be used in accordance with the invention.

For the determination of the protease activity of the mutated and subsequently PEGylated botulinum toxin the cleavage of the SNARE protein corresponding to the serotype is quantitatively recorded. The activity is then compared to the activity of (i) the native neurotoxin (of the corresponding serotype), (ii) the mutated neurotoxin with tag for the simplified isolation and/or (iii) the mutated neurotoxin without tag. The activity of the mutated neurotoxin and of the PEGylated mutated neurotoxin in accordance with the present invention is similar to the activity of the native (non-mutated) neurotoxin (that is, the biological activity, according to the definition above, of the mutated neurotoxin and of the modified neurotoxin according to the invention is at most marginally decreased in the sense of the definition given above compared to the biological activity of the neurotoxin).

The total activity of the PEGylated neurotoxin is initially determined in an ex-vivo model, the so-called diaphragm or hemidiaphragm-assay. Here the paralyzing activity is determined on a nerve-muscle preparation. In accordance with the invention the biological activity of the modified as well as of the mutated and subsequently PEGylated neurotoxin is at least 20%, preferably 30-40% or 50-70% or even 75-95% of the biological activity of the unmodified (native) protein (biological activity is also to be understood according to the definition given above).

The toxicity of the PEGylated mutated neurotoxin according to the invention can be tested in the mouse LD₅₀-assay, whereby the dose is being determined that after i.p. application is lethal for half the mice from a group.

The duration of action of the PEGylated mutated neurotoxin according to the invention is determined in vivo, also with a mouse. The period during which the muscle remains paralyzed is determined after injection of a sublethal dose of a PEGylated mutated botulinum toxin type A according to the present invention or of the corresponding native botulinum toxin in the gastrocnemius muscle of the hindpaw. The paralysis potency is classified by way of a chart. The duration of action, which is shorter in the mouse than in men, is extended by 30-150% depending on the modified botulinum toxin type A used (measured in comparison to the non-PEGylated and unmutated botulinum toxin type A).

The PEGylated mutated botulinum toxin according to the present invention can be processed in an appropriate formulation to a finished drug product, which includes a dose (or an integral multiple of the dose) in the range of the therapeutic dose (e.g., 100 LD₅₀-units per injection vial). According to a preferred embodiment the pharmaceutical composition is stabilized without addition of human serum albumin (HSA). However, it may also be stabilized with human serum albumin (HSA). In this regard the use of a HSA-free composition for the stabilization of PEGylated active protein substances as described in WO 2005/007185 is especially preferred. In accordance with a further preferred embodiment the pharmaceutical composition of the present invention is in lyophilized form or fluid. Both forms are suitable, optionally after uptake in an appropriate solvent, for i.m. injection in the muscle to be treated.

The PEGylated mutated botulinum toxin with greater stability and half life or the pharmaceutical exhibiting these can be used for therapy of various dystonia as spasmodic dystonia, laryngeal dystonia, cervical dystonia, focal hand dystonia, blepharospasm, strabism, cerebral paresis, hemifacial spasms, spasticity, spasmodic colitis, anismus, TICS, tremors, bruxism, anal fissure, achalasia, dysphagia, hyperhidrosis as well as for removal of facial wrinkles.

Additional aspects of the present investigation relate to (1) a nucleic acid, which encodes the above explicitly described modified botulinum toxin with increased stability (especially the nucleic acid is DNA); (2) a vector, comprising the nucleic acid according to (1); and (3) a host cell, comprising the vector according to (2) (especially the host cell is a prokaryotic, in particular, an E. coli host cell).

The following examples illustrate the invention in detail without limiting the invention to the aforementioned specific parameters.

EXAMPLES Example 1 Cloning and Expression of Botulinum Neurotoxin Type A (BoNT/A)

For cloning the DNA sequences of the light chain as well as of the translocation domain, chromosomal DNA was isolated from a culture of Clostridium botulinum type A (strain ATCC 3502). A coding gene fragment with modified loop sequence for the light chain of BoNT/A was obtained by using PCR amplification with the primers SEQ ID NO:1 and SEQ ID NO:2. The PCR amplificate was cloned via the restriction cleavage sites for Nco I and Bgl II into the expression plasmid pQE-H₁₀, which was derived from pQE-60 and encodes a His-tag (consisting of 10 histidine residues) at the 5′-end of the cloning site. The plasmid pQE-H₁₀-BoNT/A-L was generated by this cloning method. A coding gene fragment for the heavy chain of BoNT/A was obtained by using PCR amplification with the primers SEQ ID NO:3 and SEQ ID NO:4. It was cloned by means of the restriction cleavage sites for Stu I and Bgl II to the 3′-end of the loop sequence of the light chain in pQE-H₁₀-BoNT/A-L (plasmid pQE-H₁₀-BoNT/A). The E. coli expression strain M15[pREP4] (Qiagen) was transformed with the plasmid pQE-H₁₀-BoNT/A. The expression of the recombinant toxins was realized by a graded induction with 500 μM IPTG (final concentration) at 25° C. over night. The cells were solubilized through lysozyme and ultrasound treatment in a 50 mM phosphate buffer at pH 8.0 with 300 mM NaCl. The centrifuged lysate was incubated for 5 h at room temperature and after intermittent storage at 20° C. chromatographed on a Ni-NTA agarose column. Finally the elution fractions were dialyzed against a coupling buffer (100 mM sodium dihydrogen phosphate pH 7.5, 150 mM NaCl, 10 mM EDTA) and the protein concentration was determined. An analysis on SDS polyacrylamide gel showed that under reducing conditions two strong bands at about 50 kDa and 100 kDa as well as a weak band at 150 kDa were stained with Coomassie whereas under non-reducing conditions only one band was observed at about 150 kDa, which corresponds to the banding pattern of a botulinum neurotoxin type A in its prevailing two-chain, disulfide-bridged structure.

Example 2 Cloning and Expression of Botulinum Neurotoxin Type B (BoNT/B)

Cloning and expression of a botulinum neurotoxin type B equipped with a histidine tag (consisting of 10 histidine residues) at the N-terminus was carried out analogously to the type A toxin. Chromosomal DNA from Clostridium botulinum type B (strain Okra) as well as the primer SEQ ID NO:5 and SEQ ID NO:6 or SEQ ID NO:7 and SEQ ID NO:8 were employed for the amplification of the light and heavy chains.

Example 3 Cloning and Expression of Botulinum Neurotoxin Type C1 (BoNT/C1)

Cloning and expression of a botulinum neurotoxin type C1 equipped with a histidine tag (consisting of 10 histidine residues) at the N-terminus was done analogously to the type A toxin. Chromosomal DNA from Clostridium botulinum type C (strain 205) as well as the primer SEQ ID NO:9 and SEQ ID NO:10 or SEQ ID NO:11 and SEQ ID NO:12 were employed for the amplification of the light and heavy chain.

Example 4 Cloning and Expression of C-H₁₀-BoNT/A

To allow for a N-terminal PEGylation a cysteine residue was attached to the amino acid sequence N-terminal to the histidine tag (consisting of 10 histidine residues). This was achieved through site-directed mutagenesis in the sequence region of pQE-H₁₀-BoNT/A, which encodes the His-tag. The QuickChange Site Directed Mutagenesis Kit of Stratagene was employed. The mutagenesis reaction was carried out with the primers SEQ ID NO:13 and SEQ ID NO:14. The nucleotide exchange in the DNA sequence was verified by DNA sequencing of the isolated clones. The expression and the purification of the mutated toxin were carried out analogously to Example 1.

Example 5 PEGylation of C-H₁₀-BoNT/A

1.2 mg of C-H₁₀-BoNT/A was incubated for 30 minutes in 1 mM DTT to reduce the disulfide-bridged dimers. For separation of the reducing agent a buffer exchange to coupling buffer was carried out on a PD-10 column. The toxin solution was concentrated to 3.6 mg/ml by means of ultrafiltration. A small aliquot was incubated untreated as control sample, the remainder of the solution was mixed with a 5-fold molar excess of mPEG-Mal-5000 (Nektar Therapeutics) and rotated at ambient temperature over night. In order to avoid derivatization reactions on further cysteine residues during the sample preparation for SDS-PAGE, the PEGylation reagent was saturated with a 5-fold excess of L-cysteine. The SDS gel showed a strong additional band with slightly reduced mobility in comparison to the control batch under non-reducing conditions, while the intensity of the original toxin band at 150 kDa was significantly decreased (FIG. 2).

Example 6 In Vitro-Activity Assay

Determination of the specific protease activity (that is, the catalytic activity without binding or translocation) of the PEGylated mutated BoNT/A derivative was done in the ELISA format. For this purpose, a recombinant polypeptide was cloned, consisting in the N-terminal region of the common fusion partner glutathion-S-transferase (GST) and of a C-terminal peptide sequence, which comprises the C-terminal 17 amino acid residues of SNAP 25. These 17 amino acid residues represent the region of the substrate protein SNAP 25, in which BoNT/A specifically cleaves. After coating of a microtiter plate with the fusion protein incubation was done with H₁₀-BoNT/A as reference sample or with the mutants in their PEGylated and non-PEGylated form. The detection of the cleavage products generated in each case was done with an antibody, which specifically detects the newly formed C-terminus. The values for C-H₁₀-BoNT/A in its non-PEGylated as well as PEGylated form and for BoNT/A (reference sample) are listed in Table 3. Considering the variation limit of the assay one can observe that through the introduction of the mutation and the subsequent PEGylation the catalytic activity of the botulinum toxin was not decreased, but instead was enhanced.

TABLE 3 Relative activity [%] H₁₀-BoNT/A 100 C-H₁₀-BoNT/A 100 mPEG-C-H₁₀-BoNT/A 121.1

The specific activity was determined in protease units/ng protein.

Example 7 Determination of the Ex-Vivo Activity in the Hemidiaphragm Assay

For the determination of the total activity of the toxin derivatives, i.e., the binding of the modified neurotoxin to the receptor of the target cells and the translocation into the nerve cell and proteolysis of the SNARE substrate, the paralysis time of a nerve-muscle preparation from mouse was determined after intoxication. Again H₁₀-BoNT/A served as a reference sample. The values of the relative activity are listed in Table 4. The reduction in the activity of the modified botulinum toxins according to the present invention to 20-30% as compared to the reference sample is obviously based on the reduced ability of the toxin to bind to the target cells and on the reduced ability to translocate the toxin into the target cells.

TABLE 4 Relative activity [%] H₁₀-BoNT/A 100 C-H₁₀-BoNT/A 22.5 mPEG-C-H₁₀-BoNT/A 30

Example 8 Duration of Action of PEGylated Botulinum Toxin Type A

The duration of action of PEGylated mutated botulinum toxin (mPEG-C-H₁₀-BoNT/A) was assayed with CD 1-mice. 10 mice each received i.m. injections (2×0.05 mL) of (i) mutated, (ii) PEGylated mutated or (iii) native botulinum toxin in a dosage of 0.4 or 0.6 LD₅₀-units/mouse (in physiological saline+1 mg/mL HSA) into the gastrocnemius muscle of the hindpaw. Afterwards the paralysis of the muscle was evaluated on a daily basis by using a chart (minimal, gentle, severe paralysis). No more paralysis of the muscle was observed after 25 days in the animals treated with the native neurotoxin. In the animals treated with the mutated or PEGylated mutated botulinum toxin, the duration of action was extended for 7-20 days. 

1. A nucleic acid encoding a modified light chain of a botulinum toxin, characterized in that the modification of the light chain comprises: an extension of the chain on its N-terminus which has the structure -(C)_(n)-(tag)_(m)-(X)_(l)- in the direction from the N- to the C-terminal end, wherein C represents a cysteine residue, tag represents any tag and X represents the residue of any naturally occurring amino acid, n represents an integer from 1 to 50, m represents 0 or 1, and l represents 0 or an integer from 1 to
 50. 2. A nucleic acid encoding a modified light chain of a botulinum toxin, characterized in that the modification of the light chain comprises at least one amino acid residue, and maximally 20 amino acid residues, of the amino acid residues occurring naturally at its N-terminus mutated to a cysteine residue.
 3. (canceled)
 4. The nucleic acid according to claim 1, characterized in that the tag is a His-tag or a Strep-tag.
 5. The nucleic acid according to claim 1, characterized in that the encoded modified light chain of the botulinum toxin is a modified light chain of a botulinum toxin of the types A, B, C1, D, E, F, or G and the following conditions apply: (a) n=1, 2 or 3, m=0 or 1, l=0 or l≠0, (b) n=1, m=1, l=0; n=2, m=1, l=0; n=3, m=1, l=0; (c) n=1, m=0, l=0; n=2, m=0, l=0; n=3, m=0, l=0; (d) n=1, m=1, l≠0; n=2, m=1, l≠0; n=3, m=1, l≠0; (e) n=1, m=0, l≠0; n=2, m=0, l≠0; n=3, m=0, l≠0.
 6. The nucleic acid according to claim 1, characterized in that the extension of the encoded light chain comprises one of the following sequences: -(C)₁-(tag)₁-(X)₀-, -(C)₂-(tag)₁-(X)₀-, -(C)₃-(tag)₁-(X)₀-, -(C)₄-(tag)₁-(X)₀-, -(C)₅-(tag)₁-(X)₀-, -(C)₁-(tag)₁-(X)₁-, -(C)₂-(tag)₁-(X)₁-, -(C)₃-(tag)₁-(X)₁-, -(C)₄-(tag)₁-(X)₁-, -(C)₅-(tag)₁-(X)₁-, -(C)₁-(tag)₁-(X)₂-, -(C)₂-(tag)₁-(X)₂-, -(C)₃-(tag)₁-(X)₂-, -(C)₄-(tag)₁-(X)₂-, -(C)₅-(tag)₁-(X)₂-, -(C)₁-(tag)₁-(X)₃-, -(C)₂-(tag)₁-(X)₃-, -(C)₃-(tag)₁-(X)₃-, -(C)₄-(tag)₁-(X)₃-, -(C)₅-(tag)₁-(X)₃-, -(C)₁-(tag)₁-(X)₄-, -(C)₂-(tag)₁-(X)₄-, -(C)₃-(tag)₁-(X)₄-, -(C)₄-(tag)₁-(X)₄-, -(C)₅-(tag)₁-(X)₄-, wherein in any of these 25 sequences m can also be
 0. 7. The nucleic acid according to claim 24, characterized in that the encoded modified light chain of the botulinum toxin is a modified light chain of a botulinum toxin of the types A, B, C1, D, E, F, or G and the following conditions apply: (a) m=1, l=0, only one out of the 20 N-terminal amino acid residues is replaced by a cysteine residue, in particular only the residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; (b) m=0, l=0, only one out of the 20 N-terminal amino acid residues is replaced by a cysteine residue, in particular only the residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; (c) m=1, l≠0, only one out of the 20 N-terminal amino acid residues is replaced by a cysteine residue, in particular only the residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; (d) m=0, l≠0, only one out of the 20 N-terminal amino acid residues is replaced by a cysteine residue, in particular only the residue at position 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10; (e) m=0, l=0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2 and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6; (f) m=0, l=0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2 and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6; (g) m=1, l≠0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2 and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6; (h) m=0, l≠0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 3, 1 and 4, 2 and 4, 1 and 5, 2 and 5, 3 and 5, 1 and 6, 2 and 6, 3 and 6, or 4 and 6; (i) m=1, l=0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or 5 and 6; (j) m=0, l=0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or 5 and 6; (k) m=1, l≠0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or 5 and 6; (l) m=0, l≠0, only two out of the 20 N-terminal amino acid residues are replaced by a cysteine residue, in particular only the residues at the positions 1 and 2, 2 and 3, 3 and 4, 4 and 5, or 5 and
 6. 8. The nucleic acid according to claim 2, characterized in that the encoded modified light chain is modified such that it comprises an extension of the chain at its N-terminus, wherein the N-terminus of the extended chain comprises one of the following sequences: (a) -(tag)₁-(X)₀-BoNT(P1C), -(tag)₁-(X)₀-BoNT(F2C), -(tag)₁-(X)₀-BoNT(V3C), -(tag)₁-(X)₀-BoNT(N4C), (tag)₁-(X)₀-BoNT(K5C), (b) -(tag)₁-(X)₁-BoNT(P1C), -(tag)₁-(X)₁-BoNT(F2C), -(tag)₁-(X)₁-BoNT(V3C), -(tag)₁-(X)₁-BoNT(N4C), -(tag)₁-(X)₁-BoNT(K5C), (c) -(tag)₁-(X)₂-BoNT(P1C), -(tag)₁-(X)₂-BoNT(F2C), -(tag)₁-(X)₂-BoNT(V3C), -(tag)₁-(X)₂-BoNT(N4C), -(tag)₁-(X)₂-BoNT(K5C), (d) -(tag)₁-(X)₃-BoNT(P1C), -(tag)₁-(X)₃-BoNT(F2C), -(tag)₁-(X)₃-BoNT(V3C), -(tag)₁-(X)₃-BoNT(N4C), -(tag)₁-(X)₃-BoNT(K5C), (e) -(tag)₁-(X)₄-BoNT(P1C), -(tag)₁-(X)₄-BoNT(F2C), -(tag)₁-(X)₄-BoNT(V3C), -(tag)₁-(X)₄-BoNT(N4C), -(tag)₁-(X)₄-BoNT(K5C), wherein in any of the above listed 25 embodiments, m can also be 0 instead of
 1. 9. The nucleic acid according to claim 1, characterized in that the botulinum toxin is a botulinum toxin of type A, B, or C1.
 10. The nucleic acid according to claim 25, characterized in that the encoded modified botulinum toxin comprises at least 20%, preferably 30-40%, 50-70% or 75-95% of the biological activity of the corresponding natural (unmodified, native) botulinum toxin and has, in comparison thereto, an increased stability.
 11. The nucleic acid according to claim 1, characterized in that the encoded modified light chain is translocated in vivo into the cytosol of the motoneurons.
 12. The nucleic acid according to claim 1, characterized in that the encoded modified light chain in the cytosol of the motoneurons exhibits a higher stability than the corresponding native botulinum toxin. 13-18. (canceled)
 19. The nucleic acid according to claim 1, wherein said nucleic acid comprises DNA.
 20. The nucleic acid according to claim 19, wherein said nucleic acid is further comprised in a vector.
 21. The nucleic acid according to claim 20, wherein said vector is further comprised in a host cell.
 22. The nucleic acid according to claim 21, wherein the host cell is a prokaryotic cell.
 23. The nucleic acid according to claim 22, wherein said prokaryotic cell is an E. coli cell.
 24. The nucleic acid according to claim 2, wherein the encoded modified light chain of a botulinum toxin comprises an extension of the chain on its N-terminus which has the structure -(C)_(n)-(tag)_(m)-(X)_(l)- in the direction from the N- to the C-terminal end, wherein C represents a cysteine residue, tag represents any tag and X represents the residue of any naturally occurring amino acid, n represents an integer from 1 to 50, m represents 0 or 1, and l represents 0 or an integer from 1 to
 50. 25. The nucleic acid according to claim 1, the nucleic acid encoding a modified botulinum toxin comprising a natural heavy chain and said modified light chain.
 26. The nucleic acid according to claim 2, the nucleic acid encoding a modified botulinum toxin comprising a natural heavy chain and said modified light chain. 